Pressurized fluid containers are in widespread use for packaging and dispensing a variety of fluid products, including liquids, gases, solids and combinations thereof. Under normal operating conditions, such containers perform entirely satisfactorily. However, in the event that the contents of such containers become over-pressurized, either because of improper use, exposure to heat or for any other reason, then a violent rupture may occur. The art has provided a variety of pressure relief devices for aerosol cans to prevent explosion of the pressurized can. Many of these pressure relief devices are in the bottom of the can, while some are in the sidewall or top. The pressure relief devices that have been proposed for the bottom have one or more of three basic structures that function to relieve pressure. The first type has a valve or stopper which opens or pops when a selected pressure level is reached within the can. A second type of pressure relief device has a scored line or lines in the bottom creating one or more regions in which the thickness of the can bottom is reduced. When the pressure in the can reaches a certain level the bottom ruptures along this region. A third category of pressure relief devices provides one or more concave regions on the can bottom. This region everts when the pressure in the can reaches a selected level thereby increasing the volume of the can and reducing the pressure. Examples of some of these previously developed pressure release devices are disclosed in U.S. Pat. No. 2,795,350 (Lapin); U.S. Pat. No. 3,292,826 (Abplanalp); U.S. Pat. No. 3,512,685 (Ewald); U.S. Pat. No. 3,622,051 (Benson); U.S. Pat. No. 3,724,727 (Zundel); U.S. Pat. No. 3,786,967 (Giocomo); U.S. Pat. No. 3,815,534 (Kneusel); U.S. Pat. No. 3,826,412 (Kneusel); U.S. Pat. No. 3,831,822 (Zundel); U.S. Pat. No. 4,003,505 (Hardt); U.S. Pat. No. 4,347,942 (Jernberg et al.); U.S. Pat. No. 4,416,388 (Mulawski); and U.S. Pat. No. 4,433,791 (Mulawski). In these prior art devices, scored or coined lines of reduced material thickness are caused to fracture in response to an over-pressurization of the container contents, thereby creating vent openings. Some pressurized cans have bottoms which have both scored regions and concave regions. Other types of pressure relief devices are disclosed in U.S. Pat. No. 2,951,614 (Greene); U.S. Pat. No. 3,356,257 (Eimer); U.S. Pat. No. 3,515,308 (Hayes); U.S. Pat. No. 3,759,414 (Beard) and U.S. Pat. No. 4,158,422 (Witten et al.).
One problem with cans having score lines in the bottom is that such devices have been difficult and expensive to manufacture in the large quantities needed to fill existing commercial demands. The problem stems from the need to consistently maintain a prescribed coin depth along the line or lines surrounding either a pressure release tab or a rim of the container. This is particularly true of the device disclosed in Mulawski U.S. Pat. No. 4,433,791. When manufacturing the device from sheet steel having a thickness of 0.015″, the coined depth must be maintained within an extremely narrow range of between about 0.0015″ and 0.0025″ in order to insure that pressure is released within a range of between about 210 to 250 psi. A shallower coin depth will result in an unacceptably high pressure release, thereby presenting a risk that the container will fail in an unpredictable manner. On the other hand, a deeper coin depth may produce a prematurely low pressure release, and prompt the development of micro cracks in the remaining relatively thin membrane at the base of the coined line. These micro cracks may not always be detectable at the time of manufacture. They may occur later after the container has been filled with a pressurized product, thereby resulting in leakage and potentially costly losses. Thus, the manufacturing process must be carefully monitored with particular attention to timely equipment adjustments to compensate for tool wear, and, when appropriate, to replace worn tools. This requires frequent product sampling and testing, all of which significantly increases manufacturing costs.
Another problem that is encountered in the manufacture of pressure relief devices results from the fact that the sheet metal from which the device is formed frequently varies in thickness by a few thousandths of an inch. If the region being coined is thinner than specified, then the web at the coined region likely will be thinner than expected. A difference of only a few thousandths can result in premature rupture of the coined region.
Because of the many manufacturing problems that are encountered in making pressure relief devices having coined regions, those skilled in the art have attempted to develop pressure relief devices that do not have any coined regions. Indeed, Mulawski in U.S. Pat. No. 4,580,690 teaches away from the use of coined regions in a pressure relief device, advocating a highly effective pressure release device which is entirely free of scored or coined lines. Such a device is said to thereby obviate many of the above-described production problems associated with the prior art devices having coined regions.
Consequently, there is a need for a pressure relief device having a coined section that can be mass produced in a manner that will overcome the problems associated with the manufacture of the coined pressure relief devices of the prior art. This device should hold pressures of at least 200 psi before venting and preferably hold pressures of at least 300 psi before venting.